TOPIC: Alternator output on 1964 650

Curious - started up my 1964 650 - disconnected it from battery to measure alternator voltage... seems to go from 8 volts at idle to 25 volts at full throttle. Is this normal - or indicative of a bad alternator, rectifier or voltage regulator?

the rectifier converts AC to DC and will not controll the volts so you are OK.

What???? Your alternator produces AC voltage not DC so not sure what you were reading. Then the rectifier converts the AC to DC.
there is no regulator on the earlier motors. When the motor runs it is charging. You can blow out the older rectifiers (big finned ones not the small block style) by running without being connected to a battery so be careful.
A better way to determine charging function is to read the voltage at your battery terminals. Motor off you will read approx 12 volts which is battery voltage (duh) Once you start the motor the voltage you read at the battery terminals should be higher than the battery voltage you just read. It will be only slightly higher at idle and increase as motor speed increases 25 volts sounds way high which is why I am wondering what you really where reading. 15 to 16 volts is as high as I've ever seen.
To charge the battery the charging voltage has to be slightly higher than the Battery voltage to get the current to flow towards the battery to charge it. The system somewhat regulates as the loload from the ignition increases with RPM just as your output from the charging system increases.
The output from the earlier charging systems on these motors is not that great as back in the day about the only things running were lights and the motor ignition with a little extra to make sure the batterey stayed charged for starting.
Randy

Potential is the key here. You may have voltage, but it's the amperage that will charge the battery. Those alternators put out about 6-8 amps, if I remember correctly, which, relatively speaking, isn't a whole lot. It sounds good to me. The voltage will gradually build up to, and while at WOT. Yes, the alternator produces AC voltage, which is converted to DC through the rectifier. DO NOT run the motor with the battery disconnected!!

Potential is voltage, the alternator voltage has to be higher than the battery. Amperage output has to exceed the draw on the system so there is some residual output for the battery. In the example of 8 amp alternator output if ignition and running lights draw only 4 amps then there is still 4 amps for charging the battery. As the load increases, like you add a radio, depth finder, and other things you pretty quickly exceed the alternator output on the older motors and you will be slowly draining the battery while running even though the alternator output is withing spec.
The original Mark 75 charging system is just barely strong enough to keep the motor running and trickle charge the battery. If you idle around or go at slower speeds particularly with running lights on, my experience is it will slowly discharge the battery, voltage alwasy read above 12 volts just not enough amp output. The later year Mark 75, 78's 75A and 78A, 700's etc. had slightly more output correcting this inadaquacy but just barely. I switched the cast flywheel and stator from a later year 700 to correct this.
I think you will find the 64's had sufficent low rpm output to keep the battery happy as long as not much else besides running lights are being powered. Merc changed to a larger diameter stator for greater output shortly after they started using the flex plate flywheel design that your 650 has. If mixing and matching parts from the early 60's one needs to be sure they have not used a flex flywheel for a large stator on a small stator otherwise charging will be near nil. Conversly finding a larger stator and corresponding flywheel will improve output if you have the smaller version. I switched the 62/63 Merc 1000's I have to the larger 64 stators/flywheels to be sure of charging at slower speeds.

Correct, I think that's what I was trying to say, he has potential with that voltage output. Supply and demand, those charging systems, as you stated, can be easily overwhelmed with the addition of the accessories you mentioned. I'm not big on the accessories, running lights and ignition are my only load ( dicounting the initial starter draw ).